1. Field of the Invention
The present invention relates to a group-III nitride light-emitting device and a method for manufacturing a group-III nitride based semiconductor light-emitting device.
2. Description of the Related Art
Japanese Journal of Applied Physics Vol. 45, No. 26, 2006, pp. L659-L662 (Non-Patent Document 1) describes InGaN light-emitting diodes. These light-emitting diodes were produced on a semipolar (11-22) plane (58 degrees off the c-plane in an a-axis direction) GaN substrates. The light-emitting diodes had single quantum well structures composed of an InGaN well layers having a width of 3 nm. The following characteristics were attained. Regarding blue emission (wavelength 430 nm), the optical output was 1.76 mW, and the external quantum efficiency was 3.0%. Regarding green emission (wavelength 530 nm), the optical output was 1.91 mW, and the external quantum efficiency was 4.1%. Regarding amber emission (wavelength 580 nm), the optical output was 0.54 mW, and the external quantum efficiency was 1.3%. It was ascertained that the emission light was polarized in the [1-100] direction.
Japanese Journal of Applied Physics Vol. 46, No. 19, 2007, pp. L444-L445 (Non-Patent Document 2) describes a laser diode. The laser diode was produced on a semipolar (10-1-1) plane (62 degrees off the {000-1} plane in a m-axis direction) GaN substrate having a dislocation density of 5×106 cm−2 or less. The laser diode has an active layer having a 5-period multi-quantum well structure composed of 5-nm InGaN well layers and 8-nm GaN barrier layers. The emission wavelength was 405.9 nm, and lasing was ascertained at a threshold current density of 18 kA/cm2.
Japanese Journal of Applied Physics Vol. 44, No. 30, 2005, pp. L945-L947 (Non-Patent Document 3) describes light-emitting diodes having a 5-period multi-quantum well structures. These light-emitting diodes were produced on a semipolar (10-1-1) plane GaN template and a semipolar (10-1-3) plane GaN template. The semipolar (10-1-1) plane is inclined from a {000-1} plane at an angle of 62 degrees in the m-axis direction, and the semipolar (10-1-3) plane is inclined from a {000-1} plane at an angle of 32 degrees in the m-axis direction. In the 5-period multi-quantum well structure, the thickness of the InGaN well layer is 4 nm, and the composition of indium is 0.14. The thickness of a Si-doped GaN barrier layer is 15 nm. The emission wavelength of the light-emitting diode on the semipolar (10-1-1) plane GaN template was 439 nm. The on-wafer optical output was 0.19 mW at a current of 20 mA, and the external quantum efficiency was 0.41% at a current of 50 mA.
Applied Physics Letter Vol. 87, 2005, p. 231110 (Non-Patent Document 4) describes a light-emitting diode having a 5-period multi-quantum well structure. This light-emitting diode was produced on a semipolar (10-1-3) plane GaN template. The laser diode includes an InGaN well layer having a thickness of 4 nm and a GaN barrier layer having a thickness of 8 nm. The emission wavelength was 527.1 nm at a current of 20 mA and 520.4 nm at a current of 250 mA. The on-wafer optical output was 0.264 mW at a current of 20 mA, and the external quantum efficiency was 0.052% at a current of 20 mA.
Japanese Unexamined Patent Application Publication No. 10-135576 (Patent Document 1) describes a method for manufacturing a light-emitting semiconductor device including a group-III nitride quantum well layer formed on a nonconductive substrate. This group-III nitride quantum well layer is grown in such a way as to have a facet orientation inclined at an angle of 10 degrees or more with reference to the {0001} direction of a wurtzite crystal structure. The inclination angle can be within the range of 30 degrees to 50 degrees, within the range of 80 degrees to 100 degrees, and within the range of 130 degrees to 150 degrees.
Japanese Unexamined Patent Application Publication No. 2003-158297 (Patent Document 2) describes a semiconductor light-emitting device formed on a substrate. The semiconductor light-emitting device is formed on a {1-100} plane and a plane inclined from this plane at an off-angle within the range of −5 degrees to +5 degrees or a {11-20} plane and a plane inclined from this plane at an off-angle within the range of −5 degrees to +5 degrees.
In Non-Patent Documents 3 and 4, the GaN templates are used. In Non-Patent Documents 1 and 2 and Japanese Journal of Applied Physics Vol. 46, No. 7, 2007, pp. L129-L131 (Non-Patent Document 5), the GaN substrates are used. In Non-Patent Documents other than Non-Patent Document 3, the indium composition of the well layer is not described. Regarding Patent Document 1, a plane inclined in the a-axis direction is used, and regarding Non-Patent Documents 2 to 5, planes inclined in the m-axis direction are used. The light-emitting devices described in Non-Patent Documents 1, 2, and 5 include small-sized GaN substrates and, therefore, use of a large-diameter GaN wafer is not directed.
In Patent Document 1, attention is given to merely an effect of piezoelectric polarization, and no attention is given to the relationship between reduction in piezoelectric polarization and desired wavelength and light emission characteristics. In Patent Document 2, a desired emission wavelength is not achieved.
Since the piezoelectric polarization of the semipolar plane is smaller than the piezoelectric polarization of the (0001) plane, an increase in wavelength due to band bending of the well layer cannot be expected. Therefore, regarding the light-emitting devices including thin well layers as described in Non-Patent Documents 1 and 5, it is necessary to increase the indium composition in order to form a light-emitting device having a long emission wavelength of, for example, 410 nm or more. However, the crystal quality of the well layer having a high indium composition is degraded. As a result, the emission characteristics deteriorate. In the case where the indium composition of the well layer is relatively high, the strain included in the well layer increases as the thickness of the well layer increases. Consequently, the relationship between the thickness of the well layer and the indium composition exerts a significant influence on the emission characteristics of the active layer having the quantum well structure.